The present invention relates to apparatus for measuring weak magnetic fields, having at least one DC-SQUID comprising a planar thin film structure including Josephson junctions and a coupling coil.
It is known to use for measuring weak variable magnetic fields, for instance, in a field intensity range below 10.sup.-10 T, and in particular under 10.sup.-12 T, superconducting quantum interference elements which are known by the designation "SQUID" (Superconducting Quantum Interference Device). These elements are preferably used in medical technology, particularly in magnetocardiography, where magnetic fields of the heart in the order of about 50 pT are measured as well as in magnetoencephalography, where the magnetic fields of the brain in the order of about 0.1 pT are to be measured. The device comprises substantially a detector coil which, together with a coupling coil, forms a flux transformer. For picking up and processing signals, electronic circuitry is associated with the SQUID. Since the magnetic fields to be measured are up to six orders of magnitude smaller than external interference fields, appropriate shielding is necessary. Because of its greater sensitivity, a DC-SQUID (direct-current SQUID) can be used which contains two Josephson junctions. With an associated compensation coil, the detector coil forms a so-called gradiometer. With gradiometers of zero.sup.th, first or higher order, the biomagnetic near field which is still non-uniform in the gradiometer range can be determined selectively (Rev. Sci. Instrum. 53 (12), December 1982, pages 1815 to 1845).
A design of the DC-SQUID with the associated coupling coil as a thin-film transformer, in which the flat spiral-shaped turns of the coupling coil are arranged above the SQUID, provides good magnetic coupling and needs little space. A coupling coil with 100 turns, for instance, requires only about 2 mm.sup.2 (IEEE Transactions on Magnetics, vol. MAG-17, no. 1, January 1981, pages 400 to 403).
In order to obtain a three-dimensional field distribution, measurements must be made successively in time at different points of the region to be examined. The difficulty then arises, however, that the field data are no longer coherent over the measuring times required therefor and, in addition, measuring times are obtained which cannot be tolerated clinically. While in principle the measuring time can be shortened with known apparatus which contains several parallel measuring channels, each with an RF-SQUID (radio-frequency SQUID) (Physica 107B (1981), pages 29 and 30, North Holland Publishing Company), a gradiometer array with which at least one SQUID array is associated cannot be fabricated with this design.